This experiment
uses an orthogonal, triaxial set of sensitive linear accelerometers
to take accurate measurements of low-level (down to micro-g's) aerodynamic
accelerations along the orbiter's principal axes during initial
re-entry into the atmosphere, i.e., in the rarefied flow regime.

The aerodynamic
acceleration data from the HiRAP experiment, output on existing
ACIP channels, have been used to calculate rarefied aerodynamic
performance parameters and/or atmospheric properties pertaining
to several flights, beginning with the STS-6 mission. These flight
data support advances in predicting the aerodynamic behavior of
winged entry vehicles in the high-speed, low-density flight regime,
including free molecular flow and the transition into the hypersonic
continuum. Aerodynamic performance under these conditions cannot
be simulated in ground facilities; consequently, current predictions
rely solely on computational techniques and extrapolations of tunnel
data. For improvement or advances, these techniques depend on actual
flight data to serve as benchmarks, particularly in the transition
regime between free molecular flow and continuum flow.

Advancements
in rarefied aerodynamics of winged entry vehicles may also prove
useful in the design of future advanced orbital transfer vehicles.
Such OTVs may use aerodynamic braking and maneuvering to dissipate
excess orbital energy into the upper atmosphere upon return to lower
orbits for rendezvous with an orbiter from the space station. A
key aerodynamic parameter in the OTV design is the lift-to-drag
ratio, which is measured directly in the HiRAP experiment. Furthermore,
an OTV may require a flight-proven, sensitive onboard accelerometer
system to overcome uncertainties in the upper atmosphere. The experience
gained from the planned multiple HiRAP flights may provide valuable
test data for the development of future navigation systems. In addition,
the experiment provides data on key atmospheric properties (e.g.,
density) in a region of flight that is not readily accessible to
orbital vehicles or regular meteorological soundings.